`.dockerignore`: Use an allowlist approach for selective builds

For very selective builds, a robust strategy is to ignore everything by default (*) and then use the negation pattern (!) to explicitly include only the necessary files or directories.

Example:

```
# Ignore everything
*
# Include only what's needed
!src/
!package.json
!package-lock.json
```

Seems it has not been updated for 8 years:

https://github.com/apache/cloudstack/blob/main/.dockerignore

從儲存庫根目錄中的 .dockerignore 開始，issue 指出該檔案已過時，並將其目前的模式與提議的 allowlist 範例進行比較。驗證所得的排除項仍允許 issue 中描述的選擇性建置和所需路徑。

由索引模型根據 Issue 內容生成。